Let's break this down. We're going to add 9111 and 6618 together.

Step 1: We'll start by adding the digits 1 & 8 in column 1 and get 9.

Step 2: We'll start by adding the digits 1 & 1 in column 2 and get 2.

Step 3: We'll start by adding the digits 1 & 6 in column 3 and get 7.

Step 4: We'll start by adding the digits 9 & 6 in column 4 and get 15.
We'll write down the last digit 5 and carry the 1 to the next column.

So, 9111 + 6618 = 15729.

